1. Technical Field of the Invention
The present invention relates to digital subscriber loop (DSL) systems and, more particularly, to a method and apparatus for minimizing near-end crosstalk (NEXT) noise and reducing power consumption within a DSL system modem, transmitter and line driver.
2. Description of Related Art
Crosstalk is noise that is present on a phone line due to the electromagnetic radiation of other, closely proximate, phone lines (for example, lines located in the same cable bundle). The term “crosstalk” was originally coined to indicate the presence in a telephone receiver of unwanted speech sounds from another telephone conversation. The term has been gradually broadened in application to refer to interference between any kind of communications circuits. This kind of noise includes both near-end crosstalk (NEXT) and far-end crosstalk (FEXT) components.
With respect to digital subscriber loop (DSL) systems, it is generally accepted that the NEXT noise, as opposed to FEXT noise, presents the major source of interference. The reason for this is that FEXT noise passes through the entire DSL loop and thus its propagation loss generally is very large and in many cases the noise can simply be ignored. The opposite is true with respect to NEXT noise which undergoes little, relatively speaking, attenuation in its short propagation path. The concerns over NEXT noise remain even when the bit rate of the transmitted signal is small because idle ATM cells are inserted to fill up all the data frames of the DSL link (both upstream and downstream), and the transmission of this filler material is also a source of noise.
When DSL services are offered on different loops in the same cable bundle, it is very important to reduce and minimize NEXT noise contributed by a DSL communication on one loop with respect to the communications on other loops within the bundle. Doing so beneficially improves DSL system error rate performance and increases loop throughput.
Power consumption is also a very important factor to be managed in DSL systems. This is most commonly an issue raised with respect to the design of the DSL modem, and it applies to both the customer premises equipment (CPE) location and the central office (CO) location. A number of power concerns are recognized in the art. For example, the more power that is transmitted in a DSL system, the more likely it is that crosstalk noise will be coupled to other DSL users in the same cable bundle. It is also recognized that if a universal serial bus (USB) interface is used for an external modem at the CPE side, the power consumption of the modem is limited by the USB standard. With respect to the CO location, many DSL line cards are installed in a very limited space, and heat dissipation is a serious concern. Any reduction in power consumption in the DSL modem is therefore welcomed. Still further, power consumption is also important for laptop computers having limited capacity batteries. Finally, the use of additional bandwidth by the filler material ATM idle cells (which may lead to NEXT noise as discussed above) increases the power consumption for both of the line drivers at the CO and CPE locations without providing a substantive communications benefit.